Coal-fired electricity-generating plants may use sulfur dioxide (SO2) scrubbers to reduce the amount of SO2 that is discharged into the atmosphere. This is good environmental practice and is generally mandated by environmental regulations in most countries, including the United States, Canada, and all of Western Europe. Additionally, scrubbers can trap pollutants like mercury and other heavy metals that would otherwise be discharged with the flue gas. SO2 emissions are known to cause detrimental impacts on human health and the environment. The major health concerns associated with exposure to high ambient concentrations of SO2 include breathing difficulty, respiratory illness, and aggravation of existing cardiovascular disease. In addition to the health impacts, SO2 leads to acid deposition (e.g., acid rain) in the environment. This deposition causes acidification of lakes and streams and damage to tree foliage and agricultural crops. Furthermore, acid deposition accelerates the deterioration of buildings and monuments. While airborne, SO2 and its particulate matter (PM) derivatives contribute to visibility degradation.
Combustion of sulfur-containing fuels, such as coal and oil, results in SO2 formation. Electricity-generating plants account for the majority of SO2 emissions in the United States. The Acid Rain SO2 Reduction Program, established under Title IV of the Clean Air Act Amendments of 1990, was designed to reduce SO2 emissions from the power-generating industry. Phase I of this program began on Jan. 1, 1995, and ended on Dec. 31, 1999. In 1997, 423 electricity-generating units, affected under Phase I, emitted 5.4 million tons (4903.2×106 kg) of SO2 compared with the allowable 7.1 million tons (6446.8×106 kg). Phase II of the Acid Rain SO2 Reduction Program began on Jan. 1, 2000. To meet the requirements of this phase, some power plants may use flue gas desulfurization (FGD) technologies. Additional environmental benefits that may result from the use of these technologies are synergistic reductions in Hg and other pollutant emissions, as well as reductions in fine PM concentrations in the atmosphere.
However, FGD technologies present a number of operational challenges. For instance, FGD scrubber units are subject to large amounts of buildup that may, over time, restrict the flow of the flue gas and reduces the efficiency of the FGD scrubber.